Adhesive compositions are used extensively in bonding natural and synthetic elastomers to themselves, and to other substrates to form laminates and bonded articles. The industrial need for adhesives with the capability to bond such materials is sufficiently great, and the difficulties of achieving such a capability are sufficiently complex, that commercial acceptance of prior art adhesives has frequently represented no more than selection of the least unsatisfactory product.
Quite often, these prior art products have proven useful only for bonding a few specific elastomers to a few specific substrates, and thus are sadly lacking in versatility. Moreover, while many of these products were adequate for use with the elastomers and manufacturing processes in prevailing use at the time of their development, they have become increasingly unsatisfactory as the varieties of new and different synthetic elastomers have multiplied; as the areas in which both natural and synthetic elastomers can be advantageously used have expanded; and as the conditions of use, including temperature, flexibility, load carrying, environmental conditions and the like, have become more severe.
Adhesive compositions which have been employed in the past have included admixtures of chlorinated rubber and at least one polyalkylene polyamine adhesion promoter; mixtures of halogenated ethylene-propylene copolymer and sulfur; mixtures of chlorosulfonated polyethylene, orthoalkoxy aryl diisocyanates and dinitrosobenzene; chlorinated rubber-expoxylated novolak-epoxy resin curing agent admixture; and mixtures including chlorine-containing polymers, polyisocyanates, epoxylated novolaks, gammamethacryloxypropylthimethoxysilane, and dinitrosobenzene.
BRADLEY et al., U.S. Pat. No. 2,459,742, discloses that chlorinated rubber adhesive compositions containing at least one polyalkylene polyamine adhesion promoter can be employed for bonding natural rubber, polychloroprene, polybutadiene, butadiene-styrene copolymer, and butadiene-acrylonitrile copolymer elastomers to substrates such as metals, plastics, textiles and paper.
COLEMAN et al., U.S. Pat. No. 3,258,388, discusses the incorporation of poly-C-nitroso aromatic compounds into conventional rubber-to-metal adhesives to improve bonding. The conventional adhesives into which these compounds may be incorporated include compositions containing thermo-setting condensation polymers; polymers and copolymers of polar, ethylenically unsaturated materials; halogenated rubbers; and polyisocyanates.
DeCREASE et al. U.S. Pat. No. 3,282,883, discloses a class of adhesive compositions for bonding natural and synthetic rubbers, such as ethylene-propylene-nonconjugated diene terpolymers, neoprene, styrene-butadiene rubber, butyl rubber, halobutyl rubber, butadiene-acrylonitrile, halosulfonated polyethylene rubber, polyurethane rubber, and polyacrylate rubber. The rubbers may be bonded to themselves or to other substrates, such as metals. The adhesive compositions disclosed by DeCREASE et al. include chlorosulfonated polyethylene, orthoalkoxy aryl diisocyanates, and dinitrosobenzene.
BARKER, U.S. Pat. No. 3,824,217, discloses combining an oxime compound with an excess of a polyisocyanate compound, so that all oxime groups are reacted with isocyanate. The resulting compound may be used in compositions for bonding rubbers to primed metal substrates.
MANINO, U.S. Pat. No. 3,859,258, discloses employing the oxime-isocyanate reaction product of BARKER in a nonsulfur vulcanization system. The elastomers to which the MANINO vulcanization system may be applied can be bonded to various substrates, including metals, by curing the elastomer in situ on the substrate; priming of the substrate with a polyisocyanate is generally necessary.
WESTLEY, U.S. Pat. No. 4,581,092, discloses a cold-vulcanizable adhesive system for bonding vulcanized rubbers. The system is of particular use in creating durable seams between rubber strips or sheets. The adhesive compositions disclosed in WESTLEY include butyl rubber, a polyisocyanate compound, and at least one of a nitroso compound and an oxime compound, with the oxime compound requiring the additional presence of an oxidizing agent.
GLADDING et al., Canadian Pat. No. 729,596, discloses bonding elastomeric materials to substrates such as metals by utilizing a first adhesive layer of chlorosulfonated polyethylene; a second layer of cured rubber, such as polyisochloroprene, as an interlayer; and a third adhesive composition, including polyisocyanates and/or a polychlorinated natural rubber, to provide an interlayer of rubber-to-metal bond. This system is obviously rather cumbersome.
Russian Pat. No. 717,085 discloses a compound for modifying adhesives based on butyl rubber, which may be used in bonding rubber to metal. The patent teaches that the compound may be produced by reacting quinone dioxime with a polyisocyanate compound, in such a proportion that the ratio of oxime groups to isocyanate groups ranges from just over 1:1 up to 2:1, but does not appear to specifically teach how the compound may be used.
Experience with prior art adhesive systems in this field has revealed that, while they may be useful in bonding vulcanizable elastomers which have a low amount of residual olefinic unsaturation, they suffer from one or more drawbacks when used with more highly unsaturated elastomers; at relatively high cure temperatures; or in lengthy precure heat cycles. In many instances, reactive ingredients such as curing agents and accelerators can prematurely cure the elastomer prior to its contact with the metal substrate, causing mold fouling and, particularly at higher mold temperatures, premature curing of the adhesive. These problems become more severe as the degree of unsaturation of the elastomer increases.
Moreover, prior art systems almost universally require one or more of a dinitroso compound, an oxime compound, a polyisocyanate compound, and an oxidizing agent. The high toxicity of these ingredients poses serious handling and safety problems, and the dinitroso compounds, particularly dinitrosobenzene (DNB), exhibit fuming at relatively high cure temperatures which aggravates the problem of mold fouling. It is also impractical to incorporate an unsaturated film-forming agent into an adhesive compound which includes dinitroso compounds, or an oxime compound used in combination with an oxidizing agent, because these compounds will proceed to react with the unsaturation sites of the film-forming agent, rendering the adhesive composition unusable after a relatively short shelf life.
Thus, there remains a need for new adhesive compositions that are simple, safe, stable, and effective for bonding elastomers with a relatively high degree of unsaturation to themselves and to other substrates, especially at high vulcanization temperatures and in extended precure heat cycles.